Single-step process for the simultaneous removal of CO2, SOx and NOx from a gas mixture

ABSTRACT

A method for the removal of CO 2 , SO x  and NO x  in a single-step process is described herein. A gas mixture is directed to a carbonator. A carbonaceous material and calcium sorbent is then injected into the carbonator to remove the CO 2 , SO x  and NO x . A calciner is provided to regenerate the calcium sorbent. The unreacted carbonaceous material is used to fuel the calciner.

TECHNICAL FIELD

Exemplary embodiments of the present invention relate to the removal ofCO₂, SO_(x), and NO_(x) from a gas mixture. More particularly, exemplaryembodiments of the present invention relate to the removal of CO₂,SO_(x), and NO_(x) from a gas mixture in a single-step process.

BACKGROUND AND SUMMARY OF THE INVENTIVE CONCEPT

The increase in anthropogenic emissions has been accompanied in recentyears with an increase in research efforts to curb them. Along withcriteria pollutants such as oxides of sulphur (SO_(x)) and oxides ofnitrogen (NO_(x)), a lot of emphasis has also been placed on reducingthe CO₂ emission from stationary sources of these pollutants, such ascoal-fired power plants.

Traditional techniques for NO_(x) control include: combustion furnacemodifications such as low NO_(x) burners; flue gas recirculation;reburning of the fuel such as coal; staged combustion; post-combustionNO_(x) reduction such as selective non-catalytic reduction (SNCR) andselective catalytic reduction (SCR); and other similar technologies. Thepost-combustion NO_(x) reduction technologies convert NO to nitrogen(N₂) in a reducing atmosphere in the presence or absence of a catalyst.

Carbonaceous material has been studied extensively for removal of NOfrom combustion exhaust gases. The carbonaceous material such as coal orchar provides a reducing atmosphere for the NO_(x) gas, by gettingoxidized to CO or CO₂, and reducing NO_(x) to N₂. The presence of oxygen(O₂) in the gaseous mixture enhances the NO reduction. The C-oxygenreaction is favored over C—NO reaction; however, the C-oxygen reactionresults in the formation C(O) and C(O₂) complexes, which when desorbedfrom the surface, result in active sites on the carbon surface. Theseactive sites are then used for the C—NO dissociative chemisorption,which subsequently leads to the escape of the adsorbed N as gaseous N₂.Therefore, the O₂ present in coal combustion flue gas aids the reductionof NO_(x).

Further, carbonaceous material impregnated with alkali and alkalineearth metals (such as Na, K, Ca) and some transition metals (Cu, Ni, Co,Fe) is known to catalyze NO reduction by carbonaceous material. Themineral matter present inherently in coal char also catalyzes the NOreduction. The metal oxides provide binding sites for the oxygen whichfacilitates the reaction between C and O. Thus, there exists a multitudeof research on several parameters—the different types of metal oxidespresent in the carbonaceous material such as char, amount of theirloading, effect of the various other gaseous species involved such asSO_(x), O₂, CO₂, etc. on the overall NO reduction.

NO_(x) reduction by using carbonaceous materials derived from differentcoal types (bituminous, lignite, etc.) has been extensively studied atThe Ohio State University (“OSU”). The OSU-patented CARBONOX process wasdeveloped as a result of these studies, and was successfullydemonstrated at the pilot scale.

OSU's recent research efforts have also led to the development of theCarbonation-Calcination Reaction (CCR) Process for removal of CO₂ andSO₂ from coal-combustion flue gas. The CCR Process makes use of acalcium-based sorbent to simultaneously capture CO₂ and SO₂. In thisprocess, calcium oxide (CaO) reacts with CO₂ to form CaCO₃. The CaCO₃ isthen decomposed in another reactor to release high-purity CO₂ forsequestration and regenerate the CaO. CaO also reacts with SO₂ in thepresence of O₂ to form CaSO₄. Since CaSO₄ does not decompose at the CCRoperating conditions, a purge stream of solids is maintained to avoidCaSO₄ build-up in the solids loop. The CCR Process has also beendemonstrated at 120 kWth subpilot scale.

The novel invention described herein was successful in combining the twotechnologies—CARBONOX and CCR—to form a novel process for thesimultaneous removal of CO₂, SO_(x) and NO_(x) from a gas mixture ingeneral and coal-combustion flue gas in particular. In this novelprocess, a calcium sorbent and a carbonaceous material (like char, etc)will be contacted with the flue gas in a single reactor at anappropriate temperature and hence, simultaneous removal of CO₂, SO_(x)and NO_(x) will be achieved in a single step. NO_(x) reduction can beensured by the addition of excess carbonaceous material, and theunreacted carbonaceous material will be used as a fuel in the secondreactor to drive the endothermic regeneration reaction of the calciumsorbent.

Exemplary embodiments according to the inventive concept are anadvancement over the prior art. As stated herein, embodiments of theinventive concept combines the removal of CO₂, SO_(x) and NO_(x) into asingle step. In the exemplary process of the inventive concept, acarbonaceous material is introduced into a carbonator. A sorbent, metaloxide preferably CaO, is also introduced into the carbonator. Theproduct of the carbonator is then fed to a particle collection devicewherein the clean flue gas is separated from the solids (CaCO₃, CaSO₄,unreacted char, and unreacted sorbent (CaO)). The solids are thendirected to a calciner. The unreacted char will be combusted inoxy-combustion mode to supply heat for the endothermic sorbentregeneration reaction.

After the calciner, another PCD is provided that is used to separate thehigh-purity CO₂ stream from the solids exiting the calciner. Theregenerated sorbent exiting the calciner may proceed directly to thecarbonator or the regenerated sorbent may be directed to a hydratorbefore entering the carbonator. A purge and make-up stream may also beused in the inventive process.

The introduction of carbonaceous material into the carbonator alsoprovides advantages over the prior art. Specifically, the excesscarbonaceous material added to the carbonator can be used as fuel in thecalciner. This reduces the coal requirement due to the heat produced bythe combustion of the excess carbonaceous material. In addition, theexothermicity of carbonaceous material combustion also enables lowerinlet flue gas temperatures resulting in greater heat recovery in thesteam turbine cycle prior to the carbonator. Accordingly, not only areembodiments of the inventive concept removing CO₂, SO_(x) and NO_(x)simultaneously, but the carbonaceous material is helping to increase theefficiency of the calciner by providing fuel.

BRIEF DESCRIPTION OF THE DRAWINGS

In addition to the features mentioned above, other aspects of thepresent invention will be readily apparent from the followingdescriptions of the drawings and exemplary embodiments, wherein likereference numerals across the several views refer to identical orequivalent features, and wherein:

FIG. 1 is a schematic depiction of the CCR process;

FIG. 2 is a schematic depiction of the CCR process having a hydrator toassist in the regeneration of the sorbent;

FIG. 3 is a schematic depiction of an exemplary embodiment according tothe inventive concept, wherein carbonaceous material and calcium sorbentare introduced into the carbonator for the simultaneous removal of CO₂,SO_(x) and NO_(x);

FIG. 4 is a schematic depiction of an exemplary embodiment according tothe inventive concept, wherein carbonaceous material and calcium sorbentare introduced into the carbonator for the simultaneous removal of CO₂,SO_(x) and NO_(x), and having a hydrator; and

FIG. 5 is a diagram evidencing the simultaneous removal of NO, SO₂, andCO₂ from a simulated gas mixture using the exemplary method of theinventive concept described herein.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENT(S)

In the CCR Process previously developed and patented by OSU, CaO entersthe carbonator which operates in the temperature range of 500-700° C.(depending on the CO₂ concentration in the flue gas and the extent ofCO₂ removal required). In the carbonator, CO₂ and SO₂ are removed by CaOas per the following reactions:CaO+CO₂→CaCO₃CaO+SO₂+½O₂→CaSO₄

The gas-solid mixture from the carbonator is sent to a particle capturedevice (“PCD”) to separate the clean flue gas (i.e. flue gas minus CO₂and SO₂) from the solids. The solids, which are a mixture of CaCO₃,CaSO₄ and (unreacted) CaO, are sent to the calciner. In the calciner(operating at >850° C.), CaO is regenerated as per the followingreaction:CaCO₃→CaO+CO₂

The above reaction is endothermic and the required energy (heat) issupplied by combusting a fuel (coal, etc.) in an O₂-enrichedenvironment. Combustion in an O₂-enriched environment (also called‘oxy-combustion’) is necessary to generate high-purity CO₂ in the gasphase, in the calciner. CaSO₄ remains unaffected in the calciner. Toavoid the build-up of CaSO₄ (and other inert material like coal-ash,etc.), a solid purge stream is maintained at any appropriate location inthe CCR Process. Another PCD is used to separate the high-purity CO₂from the regenerated calcium sorbent at the exit of the calciner. Theregenerated sorbent (mainly CaO) is recycled back to the carbonator. Amake-up stream (consisting of fresh calcium sorbent) is necessary toaccount for the purged solids. FIG. 1 shows the CCR Process.

FIG. 2 depicts the CCR Process with intermediate hydration. The hydratormakes the original CCR Process a 3-step process, including: carbonation;calcination; and hydration. The hydration step reactivates the sorbentevery cycle and helps to maintain the sorbent reactivity. The CCRhydrator is a high temperature (>350° C.) steam hydrator. The followingreaction occurs in the hydrator:CaO+H₂O→Ca(OH)₂

The inclusion of the hydrator adds one more (following) reaction to theset of reactions occurring in the carbonator:Ca(OH)₂→CaO+CO₂As discussed above, the exemplary method of simultaneously removing CO₂,SO₂, and NO will be described with reference to FIG. 3. In the novelprocess, a carbonaceous material 10, such as coal char, will beintroduced in the carbonator 20 along with the calcium sorbent (CaO) 30.Although coal char is discussed herein, it should be understood thatvarious types of carbonaceous materials 10 may be used, such as char,activated carbon, and other similar materials. With the introduction ofa carbonaceous material 10, such as coal char, the following reactionswill occur in the carbonator 20, resulting in the removal of CO₂, SO₂and NO:C+2NO→CO₂+N₂2C+2NO→2CO+N₂CaO+CO₂→CaCO₃CaO+SO₂+½O₂→CaSO₄The ability to remove CO₂, SO₂ and NO in a single step eliminates theneed for additional NO_(x) control or reduction systems to be installedupstream of the carbonator 20 described herein.

The post-carbonator PCD 40 will separate the clean flue gas 50 (i.e.flue gas minus CO₂, SO₂, and NO) from the solids 60—a mixture of CaCO₃,CaSO₄, (unreacted) CaO 30 and (unreacted) carbonaceous material 10.These solids will be sent to the calciner 70. Like in the original CCRProcess, developed by OSU, coal (or any other fuel) will be combusted inoxy-combustion mode to supply heat for the endothermic sorbentregeneration reaction:CaCO₃→CaO+CO₂

However, in this invention described herein, besides the main fuel(coal, etc), the carbonaceous material 10 will also act as a fuel andget combusted in the calciner 70. A second PCD 80, post-calciner 70,will be used to separate the high-purity CO₂ stream 90 from the solids.As in the CCR Process originally developed by OSU, a purge 100 andmake-up stream 110 of CaCO₃ will be incorporated in this invention too.

In other exemplary embodiments according to the inventive concept, ahydrator 120 may also be included between the second PCD 80 andcarbonator 20, as illustrated in FIG. 4. In such a configuration, it isimportant to note that the carbonaceous material 10 will not passthrough the hydrator 120 because the carbonaceous material 10 is beingadded in the carbonator 20 and is eliminated, due to combustion, in thecalciner 70. Even if some carbonaceous material 10 escapes combustion inthe calciner 70, its presence in the hydrator 120 is not expected toaffect the performance of the hydrator 120 or the process in general.

Although FIGS. 3 and 4 illustrate schematics of the exemplary system, itshould be understood that variations may be made thereto while stillmaintain the novel features of the exemplary embodiment. In onemodification the carbonaceous material 10 and sorbent 30 may be added asindependent streams to the carbonator 20, or alternatively thecarbonaceous material 10 and the sorbent 30 may be pre-mixed beforeentering the carbonator 20. It should also be understood that sorbent 30may be loaded onto the surface of the carbonaceous material 10 in smallquantities by using methods such as solution impregnation.

The location of the purge 100 and make-up streams 110 may be moved whilemaintaining the novel features of the inventive concept. Other featuresas well may be altered such as heat integration strategies. In someembodiments, the reactors 20, 70, 120 may be operated under pressure,but atmospheric pressure operation will be the most likely andinexpensive option. The oxygen environment may also be controlled in thecarbonator 20. The flue gas composition may be modified upstream of thecarbonator 20 to control or limit the O₂ concentration so as to inhibitformation of unwanted species.

Directing attention to FIG. 5, a diagram is provided evidencing thesimultaneous removal of NO, SO₂, and CO₂ from a gas mixture using theinventive method described herein. To determine the effectiveness of theexemplary method at removing unwanted components an experiment wasdeveloped; the results of which are shown in FIG. 5. To determine theeffectiveness of the inventive method a fixed bed experiment was set-upwhere a gas mixture containing NO, SO₂, and CO₂ (similar to a flue gasstream in a power plant) was exposed to calcium sorbent and pulverizedlignite coal char. The experiment had the following inletconcentrations: 13% CO₂; 1800 ppm NO; 3050 ppm SO₂; and 1.5% O₂. Thecalcium to char loading was 10:1 by weight, and the experiment wasconducted at 650° C.

As shown in FIG. 5, through the addition of both calcium sorbent andcarbonaceous material nearly all the NO, SO₂, and CO₂ was removed fromthe outlet for approximately 37 minutes. At such time the sorbent andcoal char became saturated, allowing the CO₂ and NO to pass through thefixed bed unreacted. As illustrated by this experiment, the addition ofboth calcium sorbent and carbonaceous material into the carbonatorprovides for the simultaneous removal of NO, SO₂, and CO₂ from a gasmixture such as a flue gas stream of a power plant.

While certain embodiments of the present invention are described indetail above. The scope of the invention is not to be considered limitedby such disclosure, and modifications are possible without departingfrom the spirit of the invention as evidenced by the following claims.

What is claimed is:
 1. A method of removing unwanted components of a gasmixture, comprising: providing a carbonator; directing said gas mixtureinto said carbonator; introducing a carbonaceous material and calciumsorbent into said carbonator; removing CO₂, SO_(x), and NO_(x) from saidgas mixture simultaneously in said carbonator; providing a calciner incommunication with said carbonator; regenerating said calcium sorbent insaid calciner; and using an unreacted portion of said carbonaceousmaterial as fuel for said calciner.
 2. The method of claim 1, furthercomprising operating said carbonator in a range of about 500 to about700° C.
 3. The method of claim 1, further comprising operating saidcalciner at a temperature of greater than 850° C.
 4. The method of claim1, further comprising: providing a hydrator; directing regeneratedsorbent from said calciner to said hydrator; and hydrating saidregenerated sorbent.
 5. The method of claim 4, further comprisingoperating said hydrator at a temperature greater than 350° C.
 6. Themethod of claim 1, further comprising providing a first particlecollection device for the separation of a clean flue gas stream.
 7. Themethod of claim 1, further comprising providing a second particlecollection device for the separation of a high-purity CO₂ stream fromthe regenerated sorbent from said calciner.
 8. A method of removingunwanted components of a gas mixture, comprising: providing acarbonator, a calciner, and a hydrator; directing said gas mixture intosaid carbonator; introducing a carbonaceous material into saidcarbonator; directing a calcium sorbent into said carbonator, saidcalcium sorbent is CaO; reacting said carbonaceous material and saidcalcium sorbent with said gas mixture in said carbonator tosimultaneously remove CO₂, SO_(x), and NO_(x) from said gas mixture,resulting in a clean flue gas stream and solids; directing said cleanflue gas stream and said solids to a first particle collection deviceprovided downstream of said carbonator; separating a clean flue gasstream from said solids in said first particle collection device, saidsolids comprising, CaCO₃, CaSO₄, unreacted carbonaceous material, andunreacted calcium sorbent; directing said solids from said firstparticle collection device to said calciner; calcining said solids toproduce to produce a high-quality CO₂ stream and said calcium sorbent;using said unreacted portion of said carbonaceous material as fuel forsaid calciner; directing said high-quality CO₂ stream and said calciumsorbent to a second particle collection device; separating saidhigh-purity CO₂ stream and said calcium sorbent; directing said calciumsorbent to a hydrator; hydrator said calcium sorbent in said hydrator;and directing said sorbent from said hydrator to said carbonator.
 9. Themethod of claim 8, further comprising operating said carbonator in arange of about 500 to about 700° C.
 10. The method of claim 8, furthercomprising operating said calciner at a temperature of greater than 850°C.
 11. The method of claim 8, further comprising operating said hydratorat a temperature greater than 350° C.
 12. The method of claim 8, furthercomprising mixing said carbonaceous material and said calcium sorbentbefore introduction to said carbonator.
 13. A method of removingunwanted components of a gas mixture, comprising: introducing acarbonaceous material and a calcium sorbent into a carbonator;introducing a gas mixture into said carbonator; reacting said gasmixture with said carbonaceous material and said calcium sorbent in saidcarbonator to simultaneously remove CO₂, SO_(x), and NO_(x) from saidgas mixture; directing solid products from said carbonator to a providedcalciner; using an unreacted portion of said carbonaceous material asfuel for said calciner; and calcining a portion of said solid productsto form a regenerated calcium sorbent.
 14. The method of claim 13,further comprising operating said carbonator in a range of about 500 toabout 700° C.
 15. The method of claim 13, further comprising operatingsaid calciner at a temperature of greater than 850° C.
 16. The method ofclaim 13, further comprising: providing a hydrator; directingregenerated calcium sorbent from said calciner to said hydrator; andhydrating said regenerated sorbent.
 17. The method of claim 16, furthercomprising operating said hydrator at a temperature greater than 350° C.18. The method of claim 13, further comprising providing a firstparticle collection device for the separation of a clean flue gasstream.
 19. The method of claim 13, further comprising providing asecond particle collection device for the separation of a high-purityCO₂ stream from the regenerated calcium sorbent.